A photonic integrated circuit (PIC) or integrated optical circuit is a device that integrates multiple photonic functions and as such is analogous to an electronic integrated circuit. The major difference between the two is that a photonic integrated circuit provides functions for information signals imposed on optical carrier waves. The material platform most commercially utilized for photonic integrated circuits is indium phosphide (InP), which allows for the integration of various optically active and passive functions on the same chip. Although many current PICs are realized in InP platforms, there has been significant research in the past decade in using silicon rather than InP for the realization of PICs, due to some superior characteristics as well as superior processing capabilities for the former material, that leverage the investment already made for electronic integrated circuits.
The biggest drawback in using silicon for PICs is that it is an indirect bandgap material which makes it hard to provide electrically pumped sources. This problem is generally solved by assembling PICs comprising two or more chips made from dissimilar materials in separate processes. Such an approach is challenging due to a need for very fine alignment, which increases packaging costs and introduces scaling limitations. Another approach to solving the bandgap problem is to bond two dissimilar materials and process them together, removing the need for precise alignment and allowing for mass fabrication. In this disclosure, we use the term “hybrid” to describe the first approach that includes precise assembly of separately processed parts, and we use the term “heterogeneous” to describe the latter approach of bonding two materials and then processing the bonded result, with no precise alignment necessary.
To transfer the optical signal between dissimilar materials, the heterogeneous approach utilizes tapers whose dimensions are gradually reduced until the effective mode refractive indices of dissimilar materials match and there is efficient power transfer. This approach generally works well when materials have similar refractive indices as is the case with silicon and InP. In cases where there is larger difference in effective indices, such as between e.g. SiN and InP or GaN, the requirements on taper tip dimensions become prohibitive limiting efficient power transfer. Specifically, extremely small taper tip widths (of the order of nanometers) may be necessary to provide good coupling. Achieving such dimensions is complex and may be cost prohibitive.
Although InP and silicon-based PICs address many current needs, they have some limitations; among them the fact that the operating wavelength range is limited by material absorption increasing the losses, and the fact that there is a limit on the maximum optical intensities and consequently optimal powers that a PIC can handle. To address these limitations, alternate waveguide materials have been considered, such as SiN, TiO2, Ta2O5, AlN or others. In general, such dielectric waveguides have higher bandgap energies which provides better high-power handling and transparency at shorter wavelength, but, in general such materials also have lower refractive indices. E.g. SiN with bandgap of ˜5 eV has refractive index of ˜2, AlN has bandgap of ˜6 and refractive index of around ˜2, and SiO2 with bandgap of ˜8.9 eV has refractive index of ˜1.44. For comparison, the refractive index of GaAs and InP is >3. This makes the tapered approach challenging.
The alternative hybrid approach suffers from the drawbacks already mentioned above, namely the need for precise alignment, and correspondingly complex packaging and scaling limitations.
There remains, therefore, a need for a method that provides efficient optical coupling between materials (such as, for example, the III-V materials mentioned above, used for active devices, and simple dielectric materials used for waveguides) with dissimilar refractive indices, without requiring prohibitively narrow taper tips. This would allow for scalable integration of materials for the realization of PICs. Ideally, PICs made by such a method would operate over a wide wavelength range from visible to IR and be able to handle high optical power compared to typical Si-waveguide-based PICs.
In this disclosure we call a device or a region of a device active if it is capable of light generation, amplification, modulation and/or detection.